The invention relates to boronic compounds, in particular to novel diboron derivatives and organic boronic acid derivatives prepared therefrom. The invention also relates to processes for the preparation of these derivatives. These diboron derivatives and organic boronic acid derivatives are useful intermediates in processes for covalently linking organic compounds.
Processes for forming covalent bonds between organic compounds, both inter- and intra-molecular, are of particular importance to the synthetic organic chemist. Many such reactions are known, each requiring its own special reaction conditions, solvents, catalysts, ring activating groups etc. Some known types of coupling reactions include the Grignard reaction, Heck reactions and Suzuki reactions (N. Migaura and A. Suzuki, Chem. Rev. 1995, 95, 2457-2483).
Substituted bi- and tri-aryl compounds are of great interest to the pharmaceutical and agrochemical industries. A great number of these compounds have been found to possess pharmaceutical activity, while others have been found to be useful herbicides. There is also interest from the polymer industry in polymers prepared by the linking together of organic compounds.
Conventional methods for covalently linking aromatic rings, such as by reaction of an appropriate Grignard reagent, involve harsh conditions and are not suitable for aromatic rings with active hydrogen containing substituents. Substituents with active hydrogen atoms also can become involved in unwanted side reactions leading to undesirable products. Such substituents need to be protected prior to reaction. Boronic acid derivatives required for the Suzuki reaction are traditionally synthesized through highly reactive organo metallic intermediates.
In view of the severity of the reaction conditions the range of substituents which could be present during the linking reaction was considerably limited, and the range of useful reaction media (solvents) was restricted to those which can be expensive, difficult to remove and/or toxic.
A difficulty associated with the known coupling reactions is the limited control of the functionality of the products, leading to complex mixtures which can be difficult to separate.
Some known diboron derivatives are relatively unstable compounds, which decompose readily in aqueous solution or on exposure to air. For this reason, and a perceived difficulty in making the compounds, their use in chemical reaction is relatively unexplored.
It has now been found that diboron derivatives can be quite stable and useful in the preparation of organic boronic acid derivatives, and that properties of the diboron derivatives can be adjusted to suit particular reaction conditions or to provide particular products by selection of appropriate substituents.
Accordingly in a first aspect of the present invention there is provided a diboron derivative of formula (I) 
where R1, R2, R3 and R4 are each independently selected from the group consisting of optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, and a group of the formula xe2x80x94(R5Q)mR6 where Q is selected from O, S, NR7, optionally substituted arylene and optionally substituted cycloalkylene, m is an integer from 1 to 3, the or each R5 is independently an optionally substituted C1-C3 alkylene, R6 is C1-C3 alkyl or hydrogen and R7 is hydrogen or C1-C12 alkyl; and
each X is independently selected from O, S(O)n and NR7, where n is an integer of 0 to 3 and R7 is hydrogen or C1-C12 alkyl, or one or more of xe2x80x94NR1R7, xe2x80x94NR2R7, xe2x80x94NR3R7 and xe2x80x94NR4R7 represent an optionally substituted 5 or 6 membered heterocyclyl group,
provided that when each X is O and R1 to R4 are identical, R1 to R4 are not unsubstituted straight chain alkyl, phenyl or naphthyl, isopropyl, or phenyl substituted with alkyl; when each X is N(C1-C6 alkyl), R1 to R4 are not C1-C6 alkyl and xe2x80x94NR1R7, xe2x80x94NR2R7, xe2x80x94NR3R7 and xe2x80x94NR4R7 are not unsubstituted piperidyl or unsubstituted pyrolidinyl; when each X is NH, R1 to R4 are not C1-C6 alkyl or unsubstituted phenyl; and when xe2x80x94XR1 is xe2x80x94OCH3, xe2x80x94XR2 is xe2x80x94N(CH3)2 and xe2x80x94XR4 is xe2x80x94N(CH3)2, xe2x80x94XR3 is not OCH3.
In a second aspect of the invention there is provided a diboron derivative of formula (II) 
where X is independently selected from O, S(O)n and NR7 where n is an integer from 0 to 2, R7 is hydrogen or C1-C12 alkyl, and A1 and A2 are divalent groups which may be the same or different, provided that when each X is O, A1 and A2 are not unsubstituted C1-C3 alkylene, 1,1,2,2-tetramethylethylene, 2,2-dimethylpropylene,
1,2-dialkoxycarbonylethylene, 1,2 diphenylethylene, 1 phenylethylene, unsubstituted phenylene or phenylene mono- or di-substituted with C1-C4 alkyl; and when each X is S or NMe, both of A1 and A2 are not ethylene.
Preferably A1 and A2 are independently selected from optionally substituted alkylene, optionally substituted alkenylene, optionally substituted alkynylene, optionally substituted arylene, optionally substituted alkylarylene, optionally substituted cycloalkylene, optionally substituted cycloalkenylene, or a group of the formula xe2x80x94(R5Q)mR6 where Q is selected from O, S, NR7, optionally substituted arylene and optionally substituted cycloalkylene, m is an integer of 1 to 3, R5 and R6 are independently an optionally substituted C1-C3 alkylene, and R7 is hydrogen or C1-C12 alkyl. The divalent groups, A1 and A2, may include a fused 5 or 6 membered aliphatic or aromatic ring.
In a third aspect of the present invention there is provided a diboron derivative of formula (III) 
where R1 and R2 are each independently selected from the group consisting of optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, a group of the formula xe2x80x94(R5Q)mR6 where Q is selected from O, S, NR7, optionally substituted arylene and optionally substituted cycloalkylene, m is an integer of 1 to 3, the or each R1 is independently an optionally substituted C1-C3 alkylene, R6 is hydrogen or C1-C3 alkyl, and R1 is hydrogen or C1-C12 alkyl;
each X is independently selected from O, S(O)n and NR7, where n is an integer of 0 to 3 and R7 is hydrogen or C1-C12 alkyl, or one or both of xe2x80x94NR1R7 and xe2x80x94NR2R7 represent an optionally substituted 5 or 6 membered heterocyclyl group; and
A is a divalent group;
provided that when R1 and R2 are Me and each X is NMe then A is not unsubstituted ethylene.
Preferably A is independently selected from optionally substituted alkylene, optionally substituted alkenylene, optionally substituted arylene, optionally substituted cycloalkylene, optionally substituted cycloalkenylene, or a group of the formula xe2x80x94(R5Q)mR6xe2x80x94 where Q is selected from O, S, NR7, optionally substituted arylene and optionally substituted cycloalkylene, m is an integer of 1 to 3, R5 and R6 are independently an optionally substituted C1-C3 alkylene, and R7 is hydrogen or C1-C12 alkyl. The divalent group A, may include a fused aliphatic or aromatic ring or ring system.
The invention also provides a diboron derivative of formula (I) 
or a diboron derivative of formula (II) 
or a diboron derivative of formula (III) 
where R1, R2, R3 and R4 are each independently selected from the group consisting of optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, a group of the formula xe2x80x94(R5Q)mR6 where Q is selected from O, S, NR7, optionally substituted arylene and optionally substituted cycloalkylene, m is an integer of 1 to 3, the or and each R1 is independently an optionally substituted C1-C3 alkylene, R6 is C1-C3 alkyl or hydrogen, and R7 is hydrogen or C1-C12 alkyl;
each X is independently selected from O, S(O)n and NR7, where n is an integer from 0 to 3, R7 is hydrogen or C1-C12 alkyl, or one or more of xe2x80x94NR1R7, xe2x80x94NR2R7, xe2x80x94NR3R7 and xe2x80x94NR4R7 represent an optionally substituted 5 or 6 membered heterocyclyl group,
and A, A1 and A2 are divalent groups which may or may not be different, wherein said derivative has one or more chiral centres and there is an enantiomeric excess of one form.
Preferably the enantiomeric excess is greater than 80%, and more preferably greater than 90%.
In the above definitions, the term xe2x80x9calkylxe2x80x9d, used either alone or in compound words such as xe2x80x9calkenyloxyalkylxe2x80x9d, xe2x80x9calkylthioxe2x80x9d, xe2x80x9calkylaminoxe2x80x9d and xe2x80x9cdialkylaminoxe2x80x9d denotes straight chain or branched alky, preferably C1-20 alkyl. Examples of straight chain and branched alkyl include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, amyl, isoamyl, sec-amyl, 1,2-dimethylpropyl, 1,1-dimethyl-propyl, hexyl, 4-methylpentyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 1,1-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 1,2,2,-trimethylpropyl, 1,1,2-trimethylpropyl, heptyl, 5-methoxyhexyl, 1-methylhexyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl, 1,2-dimethylpentyl, 1,3-dimethylpentyl, 1,4-dimethyl-pentyl, 1,2,3,-trimethylbutyl, 1,1,2-trimethylbutyl, 1,1,3-trimethylbutyl, octyl, 6-methylheptyl, 1-methylheptyl, 1,1,3,3-tetramethylbutyl, nonyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-methyl-octyl, 1-, 2-, 3-, 4- or 5-ethylheptyl, 1-, 2- or 3-propylhexyl, decyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- and 8-methylnonyl, 1-, 2-, 3-, 4-, 5- or 6-ethyloctyl, 1-, 2-, 3- or 4-propylheptyl, undecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-methyldecyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-ethylnonyl, 1-, 2-, 3-, 4- or 5-propyloctyl, 1-, 2- or 3-butylheptyl, 1-pentylhexyl, dodecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-, 9- or 10-methylundecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- or 8-ethyldecyl, 1-, 2-, 3-, 4-, 5- or 6-propylnonyl, 1-, 2-, 3- or 4-butyloctyl, 1-2-pentylheptyl and the like.
The term xe2x80x9calkylenexe2x80x9d denotes a divalent alkyl group as defined above.
The term xe2x80x9ccycloalkylxe2x80x9d denotes cyclic alkyl groups, preferably C3-20 cycloalkyl. Examples of cycloalkyl include mono- or polycyclic alkyl groups such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl and the like.
The term xe2x80x9ccycloalkylenexe2x80x9d denotes a divalent cycloalkyl group as defined above.
The term xe2x80x9calkenylxe2x80x9d denotes groups formed from straight chain or branched alkenes including ethylenically mono-, di- or poly-unsaturated alkyl or groups as previously defined, preferably C2-20 alkenyl. Examples of alkenyl include vinyl, allyl, 1-methylvinyl, butenyl, iso-butenyl, 3-methyl-2-butenyl, 1-pentenyl, 1-hexenyl, 3-hexenyl, 1-heptenyl, 3-heptenyl, 1-octenyl, 1-nonenyl, 2-nonenyl, 3-nonenyl, 1-decenyl, 3-decenyl, 1,3-butadienyl, 1-4,pentadienyl, 1,3-hexadienyl, 1,4-hexadienyl.
The term xe2x80x9calkenylenexe2x80x9d denotes a divalent alkenyl group as defined above.
The term xe2x80x9ccycloalkenylxe2x80x9d denotes cyclic alkene groups, preferably C5-20 cycloalkenyl. Examples of cycloalkenyl include (cyclopentenyl, methyl cyclopentenyl, cyclohexenyl, cyclooctenyl, 1,3-cyclopentadienyl, 1,3-cyclohexadienyl, 1,4-cyclohexadienyl, 1,3-cycloheptadienyl, 1,3,5-cycloheptatrienyl and 1,3,5,7-cyclooctatetraenyl.
The term xe2x80x9ccycloalkenylenexe2x80x9d denotes a divalent cycloalkenyl group as defined above.
The term xe2x80x9carylxe2x80x9d is used herein in the broadest sense to refer to any aromatic ring or ring system, preferably having 3 to 20 carbon atoms. The ring or ring system may contain one or more heteroatoms selected from N, S, and O. The aromatic rings may be carbocyclic, heterocyclic or pseudo aromatic, and may be mono or polycyclic ring systems. Examples of suitable rings include but are not limited to benzene, biphenyl, terphenyl, quaterphenyl, naphthalene, tetrahydronaphthalene, 1-benzylnaphthalene, anthracene, dihydroanthracene, benzanthracene, dibenzanthracene, phenanthracene, perylene, pyridine, 4-phenylpyridine, 3-phenylpyridine, thiophene, benzothiophene, naphthothiophene, thianthrene, furan, pyrene, isobenzofuram, chromene, xanthene, phenoxathiin, pyrrole, imidazole, pyrazole, pyrazine, pyrimidine, pyridazine, indole, indolizine, isoindole, purine, quinoline, isoquinoline, phthalazine, quinoxaline, quinazoline, pteridine, carbazole, carboline, phenanthridine, acridine, phenanthroline, phenazine, isothiazole, isooxazole, phenoxazine and the like, each of which may be optionally substituted. The term xe2x80x9cpseudoaromaticxe2x80x9d refers to a ring system which is not strictly aromatic, but which is stablized by means of delocalization of xcfx80 electrons and behaves in a similar manner to aromatic rings. Examples of pseudoaromatic rings include but are not limited to furan, thiophene, pyrrole and the like.
The term xe2x80x9caliphatic ring or ring systemxe2x80x9d as used herein refers to a non-aromatic carbocyclic or heterocyclic ring or ring system, preferably having from 3 to 20 carbon atoms. The ring or ring system may have one or more double or triple bonds. Examples of suitable aliphatic rings include but are not limited to cyclobutane, cyclopentadiene, cyclohexanone, cyclohexene, spiro-[4,5-decane] and hydrogenated or partially hydrogenated aromatic rings as described above.
The term xe2x80x9carylenexe2x80x9d as used herein denotes a divalent xe2x80x9carylxe2x80x9d moiety as defined above.
As used herein, an xe2x80x9colefinicxe2x80x9d compound refers to any organic compound having at least one carbon to carbon double bond which is not part of an aromatic or pseudo aromatic system. The olefinic compounds may be selected from optionally substituted straight chain, branched or cyclic alkenes; and molecules, monomers and macromolecules such as polymers and dendrimers, which include at least one carbon to carbon double bond. Examples of suitable olefinic compounds include but are not limited to ethylene, propylene, but-1-ene, but-2-ene, pent-1-ene, pent-2-ene, cyclopentene, 1-methylpent-2-ene, hex-1-ene, hex-2-ene, hex-3-ene, cyclohexene, hept-1-ene, hept-2-ene, hept-3-ene, oct-1-ene, oct-2-ene, cyclooctene, non-1-ene, non-4-ene, dec-1-ene, dec-3-ene, buta-1,3-diene, penta-1,4-diene, cyclopenta-1,4-diene, hex-1,4,diene, cyclohexa-1,3-diene, cyclohexa-1,4-diene, cyclohepta-1,3-diene, cyclohepta-1,3,5-triene and cycloocta-1,3,5,7-tetraene, each of which may be optionally substituted. Preferably the straight chain branched or cyclic alkene contains between 2 and 20 carbon atoms.
In this specification xe2x80x9coptionally substitutedxe2x80x9d means that a group may or may not be further substituted with one or more groups selected from alkyl, alkenyl, alkynyl, aryl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloaryl, hydroxy, alkoxy, alkenyloxy, aryloxy, aryloxyalkyl, benzyloxy, haloalkoxy, haloalkenyloxy, haloaryloxy, isocyano, cyano, formyl, carboxyl, nitro, nitroalkyl, nitroalkenyl, nitroalkynyl, nitroaryl, nitroheterocyclyl, amino, alkylamino, dialkylamino, alkenylamino, alkynylamino, arylamino, diarylamino, benzylamino, imino, alkylimino, alkenylimino, alkynylimino, arylimino, benzylimino, dibenzylamino, acyl, alkenylacyl, alkynylacyl, arylacyl, acylamino, diacylamino, acyloxy, alkylsulphonyloxy, arylsulphenyloxy, heterocyclyl, heterocycloxy, heterocyclamino, haloheterocyclyl, alkylsulphenyl, arylsulphenyl, carboalkoxy, carboaryloxy mercapto, alkylthio, benzylthio, acylthio, sulphonamido, sulfanyl, sulfo and phosphorus-containing groups.
The term xe2x80x9cfused aliphatic or aromatic ringxe2x80x9d as used herein in relation to divalent groups A, A and A2 means that one or more of the bonds connecting the X moieties of the compounds of formulae I, II or III is part of an aliphatic or aromatic ring system.
As used herein the term xe2x80x9cdivalent groupxe2x80x9d refers to any group having two valencies available for bonding with another chemical moiety. Examples of suitable divalent groups include alkylene, alkenylene, cycloalkylene and the like.
The term xe2x80x9cacylxe2x80x9d as used herein refers to carbamoyl, aliphatic acyl group and acyl group is referred to as heterocyclic acyl, preferably C1-20 acyl. Examples of acyl include carbamoyl; straight chain or branched alkanoyl such as formyl, acetyl, propanoyl, butanoyl, 2-methylpropanoyl, pentanoyl, 2,2-dimethylpropanoyl, hexanoyl, heptanoyl, octanoyl, nonanoyl, decanoyl, undecanoyl, dodecanoyl, tridecanoyl, tetradecanoyl, pentadecanoyl, hexadecanoyl, heptadecanoyl, octadecanoyl, nonadecanoyl and icosanoyl; alkoxycarbonyl such as methoxycarbonyl, ethoxycarbonyl, t-butoxycarbonyl, t-pentyloxycarbonyl and heptyloxycarbonyl; cycloalkylcarbonyl such as cyclopropylcarbonyl, cyclobutylcarbonyl, cyclopentylcarbonyl and cyclohexylcarbonyl; alkylsulfonyl such as methylsulfonyl and ethylsulfonyl; alkoxysulfonyl such as methoxysulfonyl and ethoxysulfonyl; aroyl such as benzoyl, toluoyl and naphthoyl; aralkanoyl such as phenylalkanoyl (e.g. phenylacetyl, phenylpropanoyl, phenylbutanoyl, phenylisobutylyl, phenylpentanoyl and phenylhexanoyl) and naphthylalkanoyl (e.g. naphthylacetyl, naphthylpropanoyl and naphthylbutanoyl]; aralkenoyl such as phenylalkenoyl (e.g. phenylpropenoyl, phenylbutenoyl, phenylmethacryloyl, phenylpentenoyl and phenylhexenoyl and naphthylalkenoyl (e.g. naphthylpropenoyl, naphthylbutenoyl and naphthylpentenoyl); aralkoxycarbonyl such as phenylalkoxycarbonyl (e.g. benzyloxycarbonyl); aryloxycarbonyl such as phenoxycarbonyl and napthyloxycarbonyl; aryloxyalkanoyl such as phenoxyacetyl and phenoxypropionyl; arylcarbamoyl such as phenylcarbamoyl; arylthiocarbamoyl such as phenylthiocarbamoyl; arylglyoxyloyl such as phenylglyoxyloyl and naphthylglyoxyloyl; arylsulfonyl such as phenylsulfonyl and napthylsulfonyl; heterocycliccarbonyl; heterocyclicalkanoyl such as thienylacetyl, thienylpropanoyl, thienylbutanoyl, thienylpentanoyl, thienylhexanoyl, thiazolylacetyl, thiadiazolylacetyl and tetrazolylacetyl; heterocyclicalkenoyl such as heterocyclicpropenoyl, heterocyclicbutenoyl, heterocyclicpentenoyl and heterocyclichexenoyl; and heterocyclicglyoxyloyl such as thiazolylglyoxyloyl and thienylglyoxyloyl.
The diboron derivatives may be made following the method of Brotherton et al. [R. J. Brotherton, A. L. McCloskey, L. L. Peterson and H. Steinberg, J. Amer. Chem. Soc. 82, 6242 (1960); R. J. Brotherton, A. L. McCloskey, J. L. Boone and H. M. Manasevit, J. Amer. Chem. Soc. 82, 6245 (1960)]. In this process B(NMe2)3, obtained by reaction of BCl3 with NHMe2, is converted to BrB(NMe2)2 by reaction with a stoichiometric amount of BBr3. Reduction in refluxing toluene with sodium metal gives the diboron compound [B(NMe2)2]2 which, after purification by distillation, can be reacted with the alcohol (for example, pinacol or neopentanediol) in the presence of a stoichiometric amount (four equivalents) of HCl to give the desired ester product.
With many alcohols this reaction is unsatisfactory, the reaction being slow and complete removal of the amine being difficult unless anhydrous mineral acid (four equivalents) are added to the reaction.
An aspect of the present invention is the discovery that esters of tetrahydroxydiboron can be readily synthesised in near quantitative yield by the reaction of diboronic acid (tetrahydroxydiboron) with alcohols (including diols and polyols) and this reaction does not require the presence of acids. Furthermore, it has been found that the reaction of tetrahydroxydiboron with certain diols can be carried out with advantage in the presence of simple monoalcohols such as methanol or ethanol without these monoalcohols being incorporated in the final reaction products. These reactions, in the presence of monoalcohols, show that tetrahydroxydiboron esters of these monoalcohols readily undergo transesterification with diols that can form ring structures on a boron atom. The transesterification represents a further procedure for the production of esters of tetrahydroxydiboron. These reactions can be performed in a variety of solvents or mixtures thereof.
Accordingly in a further aspect of the present invention there is provided a process for the preparation of a diboronic acid ester comprising contacting diboronic acid with a suitable monoalcohol, diol or polyol for a time and under conditions such that the diboronic acid reacts with the diol or polyol to produce the diboron derivative.
Unlike the literature procedure, the process according to this aspect of the present invention lends itself readily to the synthesis of tetrahydroxydiboron esters with acid sensitive alcohols. The synthesis of tetrahydroxydiboron esters with alcohols possessing basic functionalities are difficult using the known literature method since the products obtained are partially or fully protonated on their basic functionalities. The process according to this aspect of the present invention does not have this inherent problem.
In the prior art procedure the side product, the acid salt of the amine, must be removed to obtain pure product. A major advantage of the present process is that product obtained is sufficiently pure that it can generally be used without the need for further purification.
The diboron derivatives according to the present invention are useful in the preparation of organic boronic acid derivatives, and by selection of appropriate substituents and reactants, it is possible to use the diboron derivatives to form organic boronic acid derivatives which are useful in organic coupling reactions.
The organic boronic acid derivatives are generally prepared by reaction of a diboron derivative of this invention and an organic compound in the presence of a Group VIII metal catalyst. In order to participate in such a reaction the organic compound should possess a boron reactive site.
The boron reactive site may be a halogen or halogen-like substituent on the organic compound, a carbon to carbon double or triple bond, or leaving group located in an allylic position.
In the case of halogen or halogen-like substituents, and allylic leaving groups, the diboron derivative displaces the group in a substitution reaction to form an organic boronic acid derivative. In the case of reaction with double and triple carbon-to-carbon bonds in the presence of platinum and like catalysts, the diboron compound tends to undergo an addition reaction across the double or triple bond to form products in which the boron esters are located on adjacent carbon atoms.
The terms xe2x80x9chalogen-like substituentxe2x80x9d and xe2x80x9cpseudo-halidexe2x80x9d refer to any substituent which, if present on an organic compound, may react with a diboron derivative in the presence of a Group VIII metal catalyst and base to give an organic boronic acid derivative. Preferred halogen substituents include I and Br. Cl may also be used although Cl is generally less reactive to substitution by the diboron compound. The reactivity of chloro substituted organic compounds can be increased by selection of appropriate ligands on the Group VIII metal catalyst. Examples of halogen-like substituents include triflates and mesylates, diazonium salts, phosphates and those described in Palladium Reagents and Catalysts (Innovations in Organic Synthesis by J. Tsuji, John Wiley and Sons, 1995, ISBN 0-471-95483-7).
As used herein, the term xe2x80x9cleaving groupxe2x80x9d refers to a chemical group which is capable of being displaced by a boronic acid residue. Suitable leaving groups are apparent to those skilled in the art and include halogen and halogen-like substituents.
The temperature at which the preparation of the diboronic acid esters is conducted will depend on a number of factors including the desired rate of reaction, solubility and reactivity of the reactants in the selected solvent, boiling point of the solvent, etc. The temperature of the reaction will generally be in the range of xe2x88x92100 to 200xc2x0 C. In a preferred embodiment the process is performed at a temperature between 0 and 80xc2x0, more preferably between 15 and 40xc2x0 C.
Chiral diboronic acid derivatives may be prepared from chiral starting materials or intermediates under conditions in which the chirality is preserved or they may also be prepared via the racemate in which case a separation step will be required. Separation of the enantiomers may be achieved conventionally using conventional chromatographic methods including chiral chromatography, enzymatic resolution, or using a resolving agent. The individual chiral forms are also part of the present invention.
Diboron derivatives containing a chiral centre, if there is an enantiomeric excess of one isomer, are particularly useful in the preparation of enantiomers of chiral compounds. In this regard it is possible to react a diboron compound having one or more chiral centers, and an enantiomeric excess of one enantiomer, with an organic compound having a boron reactive site to produce an organic boronic acid ester derivative in which the chirality is preserved. The chiral organic boronic acid derivative may then be reacted with another organic compound to produce a new chiral centre, the stereochemistry of which is induced by the stereochemistry of the chiral organic boronic acid derivative. Suitable organic compounds with which to react the chiral organic boronic acid derivative include aldehydes and unsymmetrical ketones, as reaction at the carbonyl produces a new chiral center. It is also possible to couple sterically hindered aromatic rings via an aromatic boronic acid derivative intermediate to produce chiral biaryl compounds in which the helical sense is maintained through restricted rotation about the bond linking the aromatic rings (also referred to as atropism).
In a particularly preferred embodiment of the invention a diboron derivative is prepared by reacting a suitable diboron reactant with a chiral diol, examples include pinanediol, diisopropyl tartrate, and sugars, such as mannose or galactose and like sugars containing cis-hydroxy groups or other hydroxy groups suitably orientated to couple with boron. The chiral diboron derivatives may then be reacted with suitable organic compounds having boron reactive sites to produce chiral organic boronic acid derivatives. These may be reacted in a stereospecific manner with an organic compound, with the formation of a new chiral center.
It is also possible to activate the boron to boron bond by selecting an R1 to R4, A, A1, or A2 substituent which is capable of further co-ordinating with one of the boron atoms. Such groups would include an electron rich substituent or atom which is capable of feeding electron density onto the boron atom. Examples of electron rich atoms include oxygen, nitrogen and sulphur.
A difficulty with using the known pinacol ester of diboronic acid to produce organic boronic acids is that it is difficult to cleave the pinacol ester to give the corresponding organic boronic acid. Other esters of this invention have been found to hydrolyse more readily than the pinacol ester. Esters containing an aromatic ring on the carbon xcex1 to the X moiety are surprisingly easy to cleave to the corresponding boronic acid. Benzyl ester derivatives are particularly useful for this purpose.
It is also possible to select substituents to improve the solubility of the diboron derivative in a particular solvent in which a subsequent reaction is to be carried out. Water solubility of the diboron compound can be increased by introducing polar groups, such as hydroxy groups, into the R1 to R4, A, A1 and A substituents. Similarly it is possible to select substituents which increase the solubility of the diboron compound in the desired organic solvent.
Many of the boronic acid ester derivatives prepared from the novel diboronic acid derivatives according to the present invention are also novel and represent a further aspect of the present invention.
These organic boronic acid derivatives may be reacted with organic compounds having one or more boron reactive sites to produce coupled products, as described above. These coupling reactions are generally conducted in the presence of a group VIII metal catalyst and a suitable base.
The process and compounds according to the invention are also useful for the preparation of reactive intermediates which are capable of taking part in further reactions or rearrangements. These reactive intermediates may be the organic boronic acid derivative or the coupled products. For example, organic boronic acid derivatives may take part in one or more of the palladium catalysed reactions of organoboron compounds described by Miyaura and Suzuki in Chem. Rev. 1995, 95 2457-2483. Examples of other types of reactions in which the diboron derivatives of the present invention are useful are described in copending applications PCT/AU98/00245 and PCT/AU98/00476.